This invention relates to an electronic control device for exposure factors in photography, such as the shutter speed (time or duration of the exposure) or the size of the diaphragm aperture. Merely as an illustrative example, the invention is illustrated as applied to a shutter of the "central" or "between-the-lens" type, but it is equally applicable to shutters of the focal plane or "curtain" type, and applicable to mechanism for controlling the size of the diaphragm aperture rather than the speed or timing of the shutter.
As well understood in the art, the shutter speed scale of a conventional photographic camera is customarily graduated logarithmically, each successive graduation representing a successive power of a base number. Usually, for example, each graduation represents twice or approximately twice the value of the next preceding graduation. The same relationship is often true in the graduated scale of the diaphragm aperture.
Electronic control of photographic shutters, both with respect to shutter speed (that is, time or duration of exposure) and with respect to diaphragm aperture, is well known in the art. Accurate control is difficult, however, in those arrangements where the electronic circuit contains a resistance which is varied by adjustment of the scale, because the scale is logarithmic while the resistance usually has linear characteristics; that is, a given movement along the scale represents a logarithmic change in value, whereas a corresponding given movement of the control member of the electrical resistance represents a linear change rather than a logarithmic change in value of the electrical resistance.
Various arrangements have been suggested for overcoming this discrepancy between the linear and logarithmic characteristics. In certain known setting means of this type, the logarithmic graduation of the values of the factors or settings is frequently obtained by means of a series of resistors, the individual resistances of which are graduated logarithmically with respect to each other. One difficulty is that this arrangement takes up a good deal of space, and in addition, this arrangement fails to permit an infinitely variable adjustment of the values.
It has also been attempted to control the situation by providing a logarithmically dimensioned variable resistor or potentiometer. The manufacture of such a logarithmic potentiometer is, however, very expensive, and the desired accuracy and setting cannot be assured over a wide control range which covers, for instance, several powers of ten.